I'm going to chime in here, as I consider myself a decent amateur photographer, something i've been doing for years, and I understand the mechanics of it very well.
In my opinion, the 2 big advantages that a DSLR has over other cameras is:
1. They have much bigger sensors, this allows them to have more pixels, and more importantly, bigger pixels.
2. Better lenses, which allow more light gathering and the ability to easily control aperture and focus.
The best place to see a difference in a DSLR and an iPhone is to take a photo in a darkened room, this is where you will see the greatest difference between them, as the large pixel size and larger lenses allows much greater light gathering ability, giving truly superior photos.
The second best (or maybe best, feel free to argue) it to try and capture sporting moments, once again better ability to gather light wins, this case in the form of being able to use a significantly shorter shutter speed (which freezes the moment) for the same overall exposure (brightness of the image). That is, you get less blur.
IF, however, you are in a very bight or well lit location, taking a photo of a stationary object, the difference start to reduce. The main advantages would then change to being able to easily focus (ie, manually focus on the scrathes) and to be able to change the aperture (this alters the depth of field, or how much is in focus away form the object in the centre of focus).
I've used both my DSLR and iPhone 4 to take photos of the swirls in my car (which I'll be buffing out soon!). The iPhone does a good job for a camera phone and easily a good enough job for on here MOST of the time. MY DSLR is easier to use though, and I have much more control, but I am a very experienced user.
My opinion, if you do this for a living, get a DSLR, even it's it's an old 6MP one, and learn to use it and take great photos of your work (and family/kids, if you have young kids go buy one, you will never regret it!!!!). An old 6MP DSLR will outperform a new 16MP point and shoot any day of the week, especially for kids running around having fun or pictures inside, ie parties. If you do this as a hobby and like putting pics on the forum, use whatever, and get into good light.
Epic post, clearly something I love, sorry if it was a bit long!